1. Field of the Invention
The present invention relates to pneumatic tires which are to be used for vehicles attaching importance to comfortability inside vehicle chambers during long touring or ordinary running. The pattern of the tread effectively suppresses pattern noise not only during straight running but also during turning.
2. Description of the Related Art
So-called directional patterns have been generally being used as tread patterns aiming at improvement of comfortability a inside vehicle chamber while sufficiently assuring wet performance and dry performance. In these directional tires, in order to prevent, production of great impact energy which would be caused when edges of blocks defined by inclined grooves strike against road during rotation of the tire and in order to assure rapid drainage of the inclined grooves, an angle of that portion of each of the inclined grooves which extends toward a central zone of a tread is set relatively small with respect to an equatorial plane of the tire under due consideration of a ground-contacting print of the tire. An angle of that portion of each of the inclined grooves which extends on a side zone of the tread is set relatively great with respect to the tire equatorial plane.
FIG. 5(a) is a directional tread pattern as viewed from a front side of the tire when the tire is fitted to a vehicle. In this tread pattern, since circumferential grooves c exhibits sufficient drainage performance in the central zone b of a treading surface a of the tread, pattern noises are to be reduced by providing inclined grooves d positioned in tire-circumferentially staggered phases on opposite sides, respectively, through the tire equatorial plane X--X or by forming a rib e on the central portion, while other performances are not sacrificed. Each side
zone f adjacent to the central zone b contributes less to a reduction in the pattern noises during straight running of the vehicle, but it greatly contributes to drainage. For this reason, a plurality of inclined grooves g are provided and spaced in the circumferential direction of the tire such that an angle .theta.o of each of the inclined grooves relative to the tire equatorial plane X--X is relatively small in a portion of the side zone near the central zone b and an angle .theta..sub.1 of each of the inclined grooves relative to the tire equatorial plane X--X is relatively large in a portion of the side zone near an end of the tread.
As shown in FIG. 5(b) showing a ground-contacting print of the tire in FIG. 5(a) by a broken line, a ground-contacting contour of the tire of this prior art greatly differs from an extending direction of the inclined grooves during straight running of the vehicle. Therefore, the pattern noises can be suppressed to a fully low level. On the other hand, the ground-contacting contour of the tire is changed to a shape deviated toward an axially outer side of turning as shown by a solid line in FIG. 5(b) during turning because of movement of the load. Consequently, the ground-contacting contour of the tire approaches the inclined grooves d and g. In addition, since a plane pressure of the tread becomes particularly greater on the axially outer side in the turning direction, great pattern noises resulting from striking of blocks defined by the inclined grooves d and g against road are suddenly produced depending upon the degree of the turning. This is more serious as the level of the pattern noises during straight running is smaller.